The Astrochemical Impact of Cosmic Rays in Protoclusters II: CI-to-H2 and CO-to-H2 Conversion Factors
Abstract
We utilize a modified astrochemistry code which includes cosmic ray attenuation in-situ to quantify the impact of different cosmic-ray models on the CO-to-H2 and CI-to-H2 conversion factors, X CO and X CI, respectively. We consider the impact of cosmic rays accelerated by accretion shocks, and show that clouds with star formation efficiencies greater than 2\% have X CO = (2.5 1)×1020 cm-2(K km s-1)-1, consistent with Milky Way observations. We find that changing the cosmic ray ionization rate from external sources from the canonical ζ ≈ 10-17 to ζ ≈ 10-16 s-1, which better represents observations in diffuse gas, reduces X CO by 0.2 dex for clusters with surface densities below 3 g cm-2. We show that embedded sources regulate X CO and decrease its variance across a wide range of surface densities and star formation efficiencies. Our models reproduce the trends of a decreased X CO in extreme cosmic ray environments. X CI has been proposed as an alternative to X CO due to its brightness at high redshifts. The inclusion of internal cosmic ray sources leads to 1.2 dex dispersion in X CI ranging from 2×1020 < X CI < 4×1021 cm-2 (K km s-1)-1. We show that X CI is highly sensitive to the underlying cosmic ray model.